﴿ قَالَ لَا يَأْتِيكُمَا طَعَامٌۭ تُرْزَقَانِهِۦٓ إِلَّا نَبَّأْتُكُمَا بِتَأْوِيلِهِۦ قَبْلَ أَن يَأْتِيَكُمَا ۚ ذَٰلِكُمَا مِمَّا عَلَّمَنِى رَبِّىٓ ۚ إِنِّى تَرَكْتُ مِلَّةَ قَوْمٍۢ لَّا يُؤْمِنُونَ بِٱللَّهِ وَهُم بِٱلْءَاخِرَةِ هُمْ كَٰفِرُونَ ﴾

“[Joseph] answered: "Ere there comes unto you the meal which you are [daily] fed, I shall have informed you of the real meaning of your dreams, [so that you might know what is to come] before it comes unto you: for this is [part] of the knowledge which my Sustainer has imparted to me. "Behold, I have left behind me the ways of people who do not believe in God, and who persistently refuse to acknowledge the truth of the life to come;”

ذَٰلِكُمَا مِمَّا عَلَّمَنِي رَ‌بِّي (This is of the knowledge my Lord has given me). He is saying that this has nothing to do with the art of divination through computation of figures and lines (Um al-jafr and 'am al-raml - not Jafar and Ramal, soothsaying or any other occult act of conjuring). Instead of all that, this is what my Rabb tells me - and I pass on the information. And this was an open miracle which is proof positive of prophethood and a major cause of confidence.